The long term objectives of this research are to understand the biogenesis of peroxisomes in molecular detail and to identify and characterize human genes that cause fatal illness due to failure of peroxisome biogenesis. Peroxisomes are nearly ubiquitous in eukaryotic cells and have several essential functions, including fatty acid catabolism and the first steps in plasmalogen biosynthesis. Several fatal inherited disorders (including Zellweger syndrome and rhizomelic chondrodysplasia punctata (RCDP)) are causes by defects in peroxisome biogenesis. The accumulated evidence demonstrates a branched pathway of peroxisomal protein import, with each branch representing an import receptor specific for one of several types of peroxisome targeting sequences (PTS), followed by s shared membrane translocation process. Great progress has been made in identifying genes that are required for the biogenesis of peroxisomes; currently 17 are known in yeast. Studies of yeast as a model organism have proven to be valuable in identifying human disease genes. For example, we have cloned and characterized Pex7p, a yeast receptor for the type 2 peroxisomal targeting signal (PTS2). Phenotypic similarity between RCDP patients' fibroblasts and yeast pex7 mutants led us to clone the human PEX7 homolog, and show that defects in this gene cause RCDP. Transformation of wild type human PEX7 into patient fibroblasts cured the biochemical defects of these cells. We have obtained considerable evidence indicating (unexpectedly) that the PTS2 receptor functions within peroxisomes, and within peroxisomes, and have formulated several hypotheses for its mechanism of action; critical of these alternatives are planned. We have discovered two novel proteins that are essential for the import of PTS2- targeted proteins into peroxisomes, and have new data suggesting the existence of others. A human homolog of one of these novel proteins has also been identified, which is predicted to be another human disease gene. These discoveries form the basis for future work. The planned studies should provide fundamental new information about an intriguing problem in cell biology, peroxisome biogenesis, and may also lead to useful therapeutic applications to human diseases.